6 research outputs found

    The SAM, Not the Electrodes, Dominates Charge Transport in Metal-Monolayer//Ga<sub>2</sub>O<sub>3</sub>/Gallium–Indium Eutectic Junctions

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    The liquid–metal eutectic of gallium and indium (EGaIn) is a useful electrode for making soft electrical contacts to self-assembled monolayers (SAMs). This electrode has, however, one feature whose effect on charge transport has been incompletely understood: a thin (approximately 0.7 nm) filmconsisting primarily of Ga<sub>2</sub>O<sub>3</sub>that covers its surface when in contact with air. SAMs that rectify current have been measured using this electrode in Ag<sup>TS</sup>-SAM//Ga<sub>2</sub>O<sub>3</sub>/EGaIn (where Ag<sup>TS</sup> = template-stripped Ag surface) junctions. This paper organizes evidence, both published and unpublished, showing that the molecular structure of the SAM (specifically, the presence of an accessible molecular orbital asymmetrically located within the SAM), not the difference between the electrodes or the characteristics of the Ga<sub>2</sub>O<sub>3</sub> film, causes the observed rectification. By examining and ruling out potential mechanisms of rectification that rely either on the Ga<sub>2</sub>O<sub>3</sub> film or on the asymmetry of the electrodes, this paper demonstrates that the structure of the SAM dominates charge transport through Ag<sup>TS</sup>-SAM//Ga<sub>2</sub>O<sub>3</sub>/EGaIn junctions, and that the electrical characteristics of the Ga<sub>2</sub>O<sub>3</sub> film have a negligible effect on these measurements

    Defining the Value of Injection Current and Effective Electrical Contact Area for EGaIn-Based Molecular Tunneling Junctions

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    Analysis of rates of tunneling across self-assembled monolayers (SAMs) of <i>n</i>-alkanethiolates SC<sub><i>n</i></sub> (with <i>n</i> = number of carbon atoms) incorporated in junctions having structure Ag<sup>TS</sup>-SAM//​Ga<sub>2</sub>O<sub>3</sub>/​EGaIn leads to a value for the injection tunnel current density <i>J</i><sub>0</sub> (i.e., the current flowing through an ideal junction with <i>n</i> = 0) of 10<sup>3.6±0.3</sup> A·cm<sup>–2</sup> (<i>V</i> = +0.5 V). This estimation of <i>J</i><sub>0</sub> does not involve an extrapolation in length, because it was possible to measure current densities across SAMs over the range of lengths <i>n</i> = 1–18. This value of <i>J</i><sub>0</sub> is estimated under the assumption that values of the geometrical contact area equal the values of the effective electrical contact area. Detailed experimental analysis, however, indicates that the roughness of the Ga<sub>2</sub>O<sub>3</sub> layer, and that of the Ag<sup>TS</sup>-SAM, determine values of the effective electrical contact area that are ∼10<sup>–4</sup> the corresponding values of the geometrical contact area. Conversion of the values of geometrical contact area into the corresponding values of effective electrical contact area results in <i>J</i><sub>0</sub>(+0.5 V) = 10<sup>7.6±0.8</sup> A·cm<sup>–2</sup>, which is compatible with values reported for junctions using top-electrodes of evaporated Au, and graphene, and also comparable with values of <i>J</i><sub>0</sub> estimated from tunneling through single molecules. For these EGaIn-based junctions, the value of the tunneling decay factor β (β = 0.75 ± 0.02 Å<sup>–1</sup>; β = 0.92 ± 0.02 nC<sup>–1</sup>) falls within the consensus range across different types of junctions (β = 0.73–0.89 Å<sup>–1</sup>; β = 0.9–1.1 nC<sup>–1</sup>). A comparison of the characteristics of conical Ga<sub>2</sub>O<sub>3</sub>/​EGaIn tips with the characteristics of other top-electrodes suggests that the EGaIn-based electrodes provide a particularly attractive technology for physical-organic studies of charge transport across SAMs

    Paramagnetic Ionic Liquids for Measurements of Density Using Magnetic Levitation

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    Paramagnetic ionic liquids (PILs) provide new capabilities to measurements of density using magnetic levitation (MagLev). In a typical measurement, a diamagnetic object of unknown density is placed in a container containing a PIL. The container is placed between two magnets (typically NdFeB, oriented with like poles facing). The density of the diamagnetic object can be determined by measuring its position in the magnetic field along the vertical axis (levitation height, <i>h</i>), either as an absolute value or relative to internal standards of known density. For density measurements by MagLev, PILs have three advantages over solutions of paramagnetic salts in aqueous or organic solutions: (i) negligible vapor pressures; (ii) low melting points; (iii) high thermal stabilities. In addition, the densities, magnetic susceptibilities, glass transition temperatures, thermal decomposition temperatures, viscosities, and hydrophobicities of PILs can be tuned over broad ranges by choosing the cation–anion pair. The low melting points and high thermal stabilities of PILs provide large liquidus windows for density measurements. This paper demonstrates applications and advantages of PILs in density-based analyses using MagLev

    Magnetic Two-Way Valves for Paper-Based Capillary-Driven Microfluidic Devices

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    This article presents a magnetically actuated two-way, three-position (+, 0, −), paper-based microfluidic valve that includes a neutral position (0)the first of its kind. The system is highly robust, customizable, and fully automated. The advent of a neutral position and the ability to precisely control switching frequencies establish a new platform for highly controlled fluid flows in paper-based wicking microfluidic devices. The potential utility of these valves is demonstrated in automated, programmed, patterning of dyed liquids in a wicking device akin to a colorimetric assay but with a programmed fluid/reagent delivery. These valves are fabricated using facile methods and thus remain cost-effective for adoption into affordable point-of-care/bioanalytical devices